Subsurface safety valves (SSSVs) are used within well bores to prevent the uncontrolled escape of well bore fluids, which if not controlled could directly lead to a catastrophic well blowout. Certain styles of safety valves are called flapper type valves because the valve closure member is in the form of a circular disc or in the form of a curved disc. These flappers can be opened by the application of hydraulic pressure to a piston and cylinder assembly to move an opening prong against the flapper. The opening prong is biased by a helical spring in a direction to allow the flapper to close in the event that hydraulic fluid pressure is reduced or lost.
FIGS. 1 and 2 illustrate a standard safety valve configuration 10 wherein a safety valve 14 is interposed in a tubing string 12. A control line 16 is used to open the valve. The valve 14 includes a tubular valve housing 18 with an axial passage 20. When hydraulic pressure is applied through port 22, the pressure forces a piston 24 to engage an axially shiftable opening prong 30. As the pressure forces the piston downward, the opening prong engages the closure member 32 and pushes the member into an open position. A spring 28 opposes the motion of the piston so that when the hydraulic pressure is released, the piston and opening prong are returned to a first position. The weight of the hydraulic fluid produces a "head" force against the piston, and thus is a factor in sizing the spring 28. In general, the pressure required to close the valve 14 is given by: EQU Pressure.sub.closing =Force.sub.spring /Area.sub.piston
Setting subsurface safety valves deeper is typically just a matter of ensuring sufficient closing pressure to offset the hydrostatic pressure acting to cause the valve to stay open. Increasing closing pressure is accomplished by increasing the Force.sub.spring or decreasing Area.sub.piston terms.
As the valve closing pressure increases, so does the valve opening pressure. The surface capacity to provide operating pressure is a combination of the pressure needed to open the valve and the internal well pressure: EQU Pressure.sub.surface =Pressure.sub.opening +Pressure.sub.well
However, the available surface operating pressure can be limited by the umbilical line used to deliver the hydraulic pressure. It is not uncommon for that limit to be approximately 10,000 psi. Thus, if the surface pressure is fixed and the well pressure increases with depth, the opening pressure decreases with depth.
For this reason, designs which operate independent of well pressure are required. Two well known designs are the dome charges safety valves and balance lines safety valves. A balance line valve 40 having a piston 48 in a housing 42 is illustrated in FIG. 3. Two hydraulic chambers are pressurized on opposite sides of the piston 48. A control line is coupled to a first port 44 while the balance line is coupled to a second port 46. Each hydraulic line is filled with the same type of fluid. Hydrostatic pressure from the well above and below the piston is equal. Thus, there is no downward force on the spring as a result of the hydrostatic pressure. The valve is operated by pressurizing the upper chamber 55 using the control line connected to the first port 44. This increases the downward force F1, displacing fluid from the lower chamber 51 and compressing the spring 50 to open the valve. Well pressure only has access to the upper seal 54.
Well pressure acts upwards on seal 52 and downwards on seal 54. Therefore, the radius 49 of the upper end of the piston 48 is equal to the radius 53 of the lower end, and pressure has no upward or downward resulting force on the piston as long as the seals 52, 54 remain intact. Control line pressure acts downward on surface area 56 while balance line pressure acts upward on surface area 58. Thus, the hydrostatic pressures on opposite sides of the piston 48 are equalized. If seal 52 fails, well pressure enters the balance pressure chamber 57, acting on surface area 58, and increasing F3. If the well pressure is great, it may be impossible to supply sufficient surface pressure to port 44 to force the opening prong downward. Thus, the safety valve fails to a closed position. If seal 54 fails, well pressure would enter the control chamber 55 and act on surface area 56 increasing F1. Without applying control line pressure, the F1 would be greater than F2+F3. This imbalance causes the valve to fail in an open position. The valve can be closed by pressuring up the balance line port 46 so that F3+F2 is greater that the well assisted F1. This is only possible if sufficient balance line pressure can be applied. Another failure mode occurs when gas in the well fluid migrates into the balance line, reducing the hydrostatic pressure applied by the balance line, i.e. reducing F3.
Another style of balance line safety valve is illustrated in FIG. 4. The valve 60 has a piston 64 captured within a housing 62 and three hydraulic chambers 68, 70, and 72, two above and one below the valve piston. Two hydraulic lines are run to the surface. Well pressure acts on seals 74, 80. Since the radius 63 of the upper end and the radius 68 of the lower end of the piston are the same, well pressure has no influence on the pressure required to displace the piston. One of the two hydraulic lines is a control line and is connected to port 77. The other hydraulic line is a balance line and is connected to the upper port 75 and the lower port 79. Control line and balance line hydrostatic pressures act on identical piston surface areas 65, 67 B-A' and B-A", so there is no net upward or downward force. If seal 74 leaks, well pressure accesses the balance line system. This pressure acts on surface area 67, boosting force F3, which with spring force F2 will overcome F1, to close the valve. If seal 76 leaks, communication between the control and balance lines will be established. F1 will always equal F3. Thus, F2 will be the only active force causing the valve to close. If seal 78 leaks, it has the same effect as seal 76 leaking. If seal 80 leaks, tubing pressure accesses the balance line system. This pressure acts to increase F3, overcoming F1 and closing the valve. Thus, if sufficient control line pressure is available and tubing pressure is relatively low, it may be possible to open the valve if upper seal 74 and/or lower seal 80 leak. Control line force F1 must be greater than the tubing assisted balance force F3 plus the spring force F2. In all modes of failure for this valve, the valve fails to a closed position.
A dome charge safety valve uses a captured gas charge. The gas charge provides a heavy spring force to achieve an increased closing pressure. However, dome charge designs are complex and require specialized manufacturing and personnel. This increases the cost and decreases the reliability of the design because numerous seals are required. Also, industry standards favor metal-to-metal (MTM) sealing systems. Gas charges require the use of elastomeric seals.
A need exists for a safety valve suitable for subsea applications and which is well pressure insensitive. Thus, it should incorporate the benefits of a balance line SSSV while overcoming the difficulties associated with gas migration into the balance line. Such a valve should also utilize MTM sealing systems for increased reliability. Finally, the improved valve should allow for the application of hydraulic pressure to close the valve in the event of a valve failure in an open position.